1. Field of the Invention
This invention relates to a method of and an apparatus for recording and/or reproducing data on or from a disc-like recording medium having a plurality of zones divided radially thereof.
2. Related Background Art
When information is to be recorded on or reproduced from rotating a disc-like recording medium, if the rotational velocity is constant, the recording density becomes rougher on the outer periphery of the recording medium because the recording density differs at the inner periphery compared with the outer periphery of the recording medium, and thus, considerable waste is caused by the lack of the effective utilization of the recording region of the recording medium. So, as a solution to this problem, there is known, for example, a modified constant angular velocity (MCAV) system, whereby a recording medium is rotated at a constant velocity and the recording-reproduction frequency is made higher on the outer periphery thereof. This system is also called the zone constant angular velocity (ZCAV) system. In such an MCAV system, the recording region of the recording medium is radially divided into a plurality of zones, and the recording-reproduction frequency is switched in the different zones. Accordingly, as the format of the recording medium, the number of sectors per track becomes greater on the outer peripheral tracks as shown in FIG. 1 of the accompanying drawings, and when the recording medium is rotated at a constant velocity the, information transfer speed becomes higher on at outer periphery.
Now, when the recording-reproduction frequency is to be switched for each zone, the clock frequency of a synthesizer is switched, but a predetermined time is required for the switching of the frequency of the synthesizer and some time is necessary before the frequency settles down at a target frequency. That is, as shown in FIG. 2 of the accompanying drawings, the synthesizer has a frequency switching response characteristic, and to switch the frequency from a first frequency of a certain zone to a second frequency of the next zone, a time T is necessary. This switching time T is related to jitter of a clock, and if an attempt is made to shorter the time T, the amount of jitter of the clock will become great and thus, there is a limit to the shortening of the switching time. The switching time T for the frequency, when converted into the amount of data of the disc, corresponds to a length approximate to one sector. Therefore, heretofore, it has been difficult to continuously record or reproduce data over two zones, and when data is to be recorded or reproduced over two zones, there has been adopted a technique of temporarily discontinuing recording or reproduction when the recording or reproduction on a zone is terminated, and waiting for one full rotation of the disc, and again starting recording or reproduction at the head of the next zone.